The present invention relates to cement compositions and methods of their use in subterranean applications. More specifically, the present invention relates to the use of a self-degrading cement composition for use in fluid loss applications.
Lost circulation involves the loss of drilling or cementing fluids into formation voids during drilling, circulation, running casing, or cementing operations. Zones of high porosity and/or permeability, rubble zones, gravel and other natural voids may all cause fluid loss into the formation. In some circumstances, lost-circulation problems are caused in depleted zones where the formation pore pressure is lower than that of the upper portion of the formation. In such cases, increases in hydrostatic pressure may fracture weak formations and lead to lost circulation. Fluid loss to thief zones may also be problematic, for example, during cementing operations where water loss to such zones may result in the formation of a dehydrated cement bridge. The concomitant lowering of hydrostatic pressure below such a bridge may cause formation gases to bubble up through the cement resulting in channeling through the cement column and up to the surface of the formation. To prevent fluid loss in these various situations, a lost circulation material (LCM) is typically employed.
LCMs are diverse in nature and include, for example, various bridging agents in granular, fiber, or flake form, crosslinkable polymers, and swellable polymers. Some LCMs may be added directly to the drilling or cement composition. LCMs and chemical products specifically designed to treat fluid loss include, for example, cellulose, almond hulls, black walnut hulls, dried tumbleweed, kenaf, paper, asphalt and both coarse and fine rice. Another method involves pumping a powdered bentonite-diesel oil pill and chasing it with water. The pill forms a semi-solid mass that may stem severe fluid loss. Bentonite may also be mixed with polymers to form a pliable gel in the presence of water. These LCMs may provide relief in moderate fluid loss situations, but many are inadequate to restore fluid circulation in situations involving severe fluid loss.
Among LCM materials, various cement compositions are also commonly employed. Fluid loss control during cementing operations, in particular, may employ rapid-set or thixotropic cement, or lightweight cement systems. However, current cement compositions employed in fluid loss prevention applications may lack sufficient compressive strength and/or may have setting times that are too long, limiting their usefulness in severe fluid loss situations. Moreover, cements employed in severe fluid loss situations may also lack degradability, which may render the cemented zone relatively impermeable, thus limiting access to the formation fluids. This lack of accessibility may necessitate further costly operations to gain access to these zones.